1. Field of the Invention
The present invention relates to a continuously variable transmission and, in particular to a continuously variable transmission for a vehicle.
2. Description of the Related Art
Vehicles are changed in driving mode from manual gear shifting to automatic gear shifting due to application of continuously variable transmissions to the vehicles. Since vehicles with automatic gear shifting are convenient in operation, fluent in acceleration, and safety in drive, they are increasingly appeal to car-purchasers. In the prior art, the continuously variable transmissions for achieving automatic gear shifting comprise hydraulic automatic transmissions and metal belt type continuously variable transmissions. These two transmissions have many disadvantages which will be described in detail as follows.
The hydraulic automatic transmissions comprise hydraulic couplings and hydraulic torque converters. A hydraulic coupling operates based on the following principle: a power source drives a pump impeller thereof to rotate so that the pump impeller drives liquid among blades of the pump impeller to rotate. As a result, energy from the power source is transmitted to the liquid so that the liquid increases in kinetic energy. When the liquid increasing in kinetic energy enters into spaces between respective blades of a runner, a part of kinetic energy contained in the liquid is transmitted to the runner which rotates slower than the pump impeller so that the runner outputs torque to the outside. In order to increase the torque output from the runner, a reactor, which rotates in one direction, is further disposed between the pump impeller and the runner, so that a hydraulic torque converter is configured.
The hydraulic torque converter can counterbalance some disadvantages of a mechanically controlled step transmission, and has some prominent advantages. For example, a vehicle with a hydraulic torque converter is simple in operation, and labor saving, as well as improves in safety and comfort of drive and in overtaking acceleration, and lowers in harmful exhaust to comply with the environmental protection requirement. Further, since the power source and a gear train are coupled flexibly, a life time of the vehicle lengthens. Finally, the vehicle has excellent adaptability to improve adaptability to various road surfaces. However, the hydraulic torque converter has the following apparent disadvantages.
1. Since the hydraulic torque converter has a small range of change in speed and a small range of change in torque so that it can not separately satisfy a usage requirement of a vehicle, it is necessarily connected in parallel or series with a mechanically controlled transmission cooperating therewith. In addition, in order to achieve an automatic gear shifting of the mechanically controlled transmission, a hydraulic or electric control system is needed to be provided. Therefore, the corresponding structure is complicated, and the manufacturing cost thereof is expensive.
2. Since the energy is transmitted by a liquid as a transmitting medium, transmission efficiency is low, and energy loss is great so that the vehicle is lowered in economic usage of fuel oil.
3. Since the transmission is complicated in structure, it is difficult to service the transmission. Therefore, a special service person that has a high service level and malfunction inspecting and analyzing ability is required for the service. As a result, service cost is high.
Chinese Patent CN No. 1136108C published on Jan. 28, 2004 discloses a hydraulic automatic transmitting system comprising a torque converter for fluidly coupling a engine and a transmission; a primary gear shifting part disposed on a first shaft; a secondary gear shifting part disposed on a second shaft; a power transmitting device (consisting of four power transmitting gears) for transmitting a rotary power from the primary gear shifting part to the secondary gear shifting part; first, second, and third brakes and clutches for controlling the primary gear shifting part and the secondary gear shifting part; and first and second one-way clutches. The torque converter consists of a pump impeller, a runner, a reactor disposed between the pump impeller and the runner. The pump impeller, the runner, and a reactor constitute a hydraulic torque converter. Although the above automatic transmitting system improves a lot over the prior art, it still has many disadvantages as mentioned above.
A metal belt type continuously variable automatic transmission operates based on the following principle: a power output from a power source is transmitted to a driving working wheel of a stepless speed change device, and the driving working wheel transmits the power to a driven wheel via a V-shaped metal belt. After that, the power is transmitted to wheels of a vehicle through an intermediate speed reducer, a main speed reducer, and a differential. A metal belt transmitting device in the transmission is the core of the transmission and comprises the driving working wheel, the driven working wheel, and the V-shaped metal belt connecting the driving working wheel and the driven working wheel. Each of the driving working wheel and the driven working wheel is composed of a stationary cone disc and a movable cone disc assembled coaxially. The stationary cone disc and the movable cone disc form a V-shaped groove with which the V-shaped metal belt engages, so that the V-shaped belt transmits the power under pressing force of the stationary cone disc and the movable cone disc. When the movable cone discs of the driving working wheel and the driven working wheel moves axially during operation, a radius of the metal belt varies, so that a transmission radio is changed. The movable cone disc moves axially by adjusting hydraulic oil within cylinders for the driving working wheel and the driven working wheel which are in turn controlled by means of a mechanical-hydraulic or electric-hydraulic control system. Since pressure of the hydraulic oil can be adjusted continuously, the transmission also can achieve stepless speed change.
With the metal belt type continuously variable automatic transmission, a vehicle has advantages that it is excellent in kinetic characteristic, convenient in operation, and high in transmission efficiency. In addition, the power source can always operate within an economic rotary speed range thereof, so that the vehicle is greatly increased in economic usage of fuel oil and is improved in exhaust. However, the transmission still has the following unsurmountable disadvantages.
1. Since the power is transmitted by means of friction between the metal belt and the driving and driven working wheels, slippage may occur between the metal belt and the driving and driven working wheels so that motive power with a large power or a large torque cannot be transmitted. Therefore, the transmission can not match a power source with a large displacement. Currently, the transmission is only used in cars with a medium or small displacement so that application range thereof is limited.
2. A vehicle with the transmission is low in starting performance. If a driver wants to abruptly accelerate, the transmission can not respond rapidly since it will take time to change diameters of the working wheels. In addition, since the vehicle needs a large starting torque, it is necessarily provided with starting devices such as multiple-disc wet clutches, electromagnetic clutches, and hydraulic torque converters. As a result, the structure of the transmission becomes very complicated.
3. It is difficult to manufacture the metal belt and the manufacturing cost is expensive. Therefore, special equipment for the manufacturing is needed, and an amount of replacement of equipment is large.
Chinese Laid-Open Application CN No. 1442623 published on Sep. 17, 2003 discloses such a metal belt type mechanically controlled frictional continuously variable transmission which has the above disadvantages.
Furthermore, U.S. Pat. No. 6,062,096 discloses a continuously variable transmission which transmits torque by utilizing rocking arms oscillating. Eccentric masses are provided at ends of the rocking arms. An input drives the eccentric masses to rotate around respective axes thereof. Centrifugal forces generated when the eccentric masses rotate drive the rocking arms to oscillate. Although the transmission overcomes some of the disadvantages occurring in the above two transmissions, it still has the following disadvantages.
1. Since the rocking arms oscillate during operation, in order to assure that an output shaft always outputs a speed in one direction, it is required that two one-way clutches which have respective lockup directions opposite to each other are disposed on sleeves directly connected with the rocking arms, and a set of reversing gears are disposed to reverse a rotary speed output from one of the one-way clutches, so as to assure that the one-way clutches output the speed in the same direction. Therefore, the output torque is output alternately through the two one-way clutches. As a result, the transmission is bulky and complicated in structure, and when the output speed is high, the oscillating rocking arms are subject to a large inertia force, thereby making the requirement for material and manufacturing accuracy of the rocking arms and bearings thereof high.
2. Since a one-way clutch is not disposed between the torque converter and a prime mover, a part of kinetic energy stored in the eccentric masses is transmitted back to the prime mover so as to cause circulation of the power, thereby affecting effective exertion of performance of the transmission.
3. Since the masses of the eccentric masses vary by changing phases of the eccentric masses so as to adjust output torque and rotary speed, additional manual or automatic controlling mechanism is required. If the manual controlling mechanism is adopted, the phases of the eccentric masses necessarily are manually adjusted, but can not be automatically adjusted. If the automatic controlling mechanism is added, a complicated feed back system is required, so that the manufacturing cost greatly increases.
4. The one way clutch at output part of the transmission is a radially engaging line-contact sprag clutch, the one way clutch having such structure not only is not sufficiently sensitive in response since the sprags are affected by centrifugal forces generated by rotation thereof, respectively, but also a large moment is required during disengagement and a large amount of energy is consumed so as to decrease the transmission efficiency of the transmission. In addition, since the sprags are in line contact with both an inter ring and an outer ring, the clutch is low in wear resistance so that it can not transmit a large torque and is short in lift time. Therefore, the lift time of the entire transmission is shortened.
U.S. Pat. No. 6,044,718, which is a continuation-in-part of U.S. Pat. No. 6,062,096, further discloses a new solution in addition to the original ones. However, the further disclosed solution is substantially similar to the others. In the further solution, the rocking arm mechanism is substantially not changed, and two one-way clutches which have respective lockup directions opposite to each other are still needed to be disposed on sleeves directly connected with the rocking arms. The further solution differs from the others in that one clutch of the one-way clutches is fitted between the sleeves of the rocking arms and a base frame. The one clutch thus limits swing of the rocking arms towards one direction so that the rocking arms only can swing intermittently in another direction. The transmission not only still has the above disadvantages, but also makes operating condition of the one-way clutch fitted between the rocking arms and the base frame worse, since the base frame is stationary, and the one-way clutch bears a large load so as to limit the swing of the rocking arms towards the one direction. As a result, the sprags of the clutch tightly wedge into the sleeves of the rocking arms and a corresponding hole of the frame, so that it is more difficult to disengage the clutch, more energy is consumed, the wear of the clutch is more severe, and thus the life time is severely affected.